Most prior art oil fryer cooking devices do not include as part of their structure means for fully filtering the oil used therein. These devices required that a separate filter apparatus be connected temporarily to the cooker to provide complete oil filtering using diatomaceous earth filters or the like. Such diatomaceous earth filters enable the harmful solids and other smaller particulate debris in the oil to be removed when the oil is drawn through the filter. Disadvantages of such devices included the fact that with a filter separate from the cooker, the filtering process required that the hot oil be manually handled during filtering, a dangerous situation especially for untrained workers. Further, some of the oil is always lost during this process. Finally, the requirements of hooking up the filter apparatus to the cooker adds significant time to the filtering process, such that it is only normally done once or twice a day. Thus, the oil is not filtered sufficiently often to prevent build-up of impurities and solids, thereby causing the oil to break down and become unusable sooner than would otherwise be possible.
Other devices have made filters an integral part of the cooker apparatus, but these devices have had the disadvantage that the diatomaceous earth is not used in the filtering process, so that the filter acts more as a strainer rather than a complete filtering device. Thus only larger particulate matter, e.g. crumbs, are removed by such filtering.
Only one other cooker apparatus is known to use diatomaceous earth as an integral part of its structure. However, this apparatus has the drawback that the earth is added to the drain pan, not to the cooking vessel, and it apparently does not include means for preventing the oil from disturbing the earth after it has been deposited onto the surface of the filter during draining of oil into the drain pan. If a portion of the surface of the filter is exposed, the oil drawn through the filter at this point is not adequately filtered, thereby reducing the effectiveness of the filtering operation.